Projection-type image display apparatuses are known in which an original image, such as a still image of a letter or a picture, or a moving image, is two-dimensionally formed on a small light valve and then magnified and projected by a projection optical system. The light valve, which may also be referred to as a spatial light modulator, is utilized in technologies such as transmissive liquid crystal display (LCD) panels and Digital Light Processing (DLP) and Liquid Crystal on Silicon (LCoS) technologies.
While DLP is currently the most dominant technology in the market place, the LCoS panel is increasing its share because of its superior contrast characteristics and higher optical utilization efficiency than the transmissive LCD panel, and because of its capability to accommodate the increasing numbers of pixels in the recent years.
Projector-type image display apparatuses include the front-projection type and the rear-projection type. In the front-projection type, an image on the light valve is magnified and projected on a projection surface, such as a reflecting screen, disposed away from the device so that the reflected light can be observed. In the rear-projection type, a transmissive screen is provided within the apparatus as a projection surface, and an image on the light valve is magnified and projected onto the screen from behind it, so that the projected image can be viewed from the front of the screen.
In either type of the image display apparatus, it is desirable to reduce the size of the apparatus. Further, by reducing the distance between the device and the projection surface, it becomes possible to realize a large screen size within the minimum space.
Patent Document 1 indicated below discloses a rear-projection apparatus in which the optical path is folded twice between the light valve and the screen, whereby an original image that is longer vertically on the light valve is projected longer horizontally on the screen.
In this technology, a planar mirror is provided as a first light-deflecting unit disposed in the optical path of a projection lens system in order to deflect the optical path in a direction perpendicular to a base surface. A second light-deflecting unit is disposed in the optical path between the projection lens system and the screen surface in order to deflect the optical path in a direction parallel to the base surface.
Patent Document 2 discloses a projection optical apparatus (oblique projection optical apparatus) having a reduced projection distance. In this technology, an intermediate image of an original image on the light valve is generated by a transmissive-refractive system. The intermediate image is refocused by a concave mirror onto the screen.
Patent Document 3 discloses that the optical path of a light flux emerging from a group of lenses is deflected by a mirror in order to fold the optical axis.
Specifically, the light rays for an image on a panel (spatial light modulator) are bent by a mirror after passing through an optical system and are further bent by another mirror before they reach a concave mirror, by which the light rays are bent and additionally bent by another mirror before the image is projected on a projection surface.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-347872
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-235516
Patent Document 3: WO2006-058884
In the technology of Patent Document 1, the projection lens system extends vertically with respect to the base plate surface due to the first light-deflecting unit. Furthermore, there is the second light-deflecting unit disposed in the optical path between the projection lens system and the screen surface.
In this structure, because the height from the base surface is large, the apparatus would have an increased height if adapted for front-projection purposes.
In the optical system according to Patent Document 2, the thickness of the apparatus as a whole in the up-down direction, when adapted for a front-projection configuration, is obviously dependent on the size (height) of the largest concave mirror. In addition, because the transmissive-refractive optical system as a whole is disposed above the concave mirror, the height of the apparatus is increased all the more.
This problem is neither so conspicuous nor serious when the optical system is used for a rear-projection application and contained in the casing of a rear-projection apparatus. However, when applied in a front-projection apparatus, the size of the apparatus becomes much more noticeable.
Furthermore, the space below the transmissive-refractive optical system provides an extensive dead space extending to the lower edge of the concave mirror. Because the optical system is disposed linearly in the projection direction, the length of the apparatus in the projection direction, i.e., the depth of the apparatus, is large, thus making it difficult to realize a compact projection apparatus (because the optical system needs to be disposed further in the back of the projected image).
While in the technology of Patent Document 3, the optical axis is bent downward by a mirror, because the panel (spatial light modulator) is located below the concave mirror, the height of the set is increased.
Additionally, because the optical path is bent between the projection lens system and the concave mirror, the size of the elements required for the deflection of the optical path increases due to reasons which will be discussed below. The position of the projection light flux as it travels from the projection lens system to the concave mirror is off the optical axis of the projection lens system, which makes the setting of the elements in the optical path deflecting portion difficult.
In order to interpose the elements for folding the optical path, the distance between the projection lens system and the concave mirror needs to be increased. As seen from the drawings of Patent Document 2 illustrating the optical path, the projection light flux from the projection lens to the concave mirror has increasingly greater angles with respect to the optical axis of the lens system as the angle of view increases. As a result, the size of the concave mirror that receives such light fluxes increases as the distance from the lens system increases, so that the cost of the concave mirror also increases in a corresponding manner.
In the technology of Patent Document 3, an intermediate image is located between the projection lens system and the concave mirror, and the optical path is bent near the intermediate image, with the folding mirror disposed where the light fluxes are concentrated. As a result, the quality of the projected image is subject to even a slight error in the shape of the folding mirror and thus tends to degrade.
An analysis by the present inventors indicates that the optical systems of the type in which an intermediate image is formed, as in Patent Documents 2 and 3, have certain characteristics in the light rays emerging from the projection lens system.
Specifically, the light ray with a large angle of view (i.e., the ray that emerges from a large height of the object) passes through a point close to the edge of a final lens, so that it is guided to the concave mirror with a large output angle.
Due to such features, the size of the final lens or a lens disposed nearby it in the projection lens system tends to increase when the optical path space is extended for the folding.
Furthermore, because the light fluxes from the lens system spread before they are incident on the concave mirror, the size of the concave mirror also tends to increase.
In Patent Documents 1 to 3, the optical path of the light ray that emerges with a large output angle from the projection lens system is bent by a folding mirror.
Such folding of the optical axis invites an increase in the size of the lenses, and prevents a decrease in the size of the apparatus.
When the folding mirror is disposed where light fluxes are focused as mentioned above, the quality of the projected image is subject to even a slight error in the shape of the folding mirror and thus tends to degrade.
Thus, in the aforementioned types of projection optical apparatuses, the decrease in size of the apparatus cannot be achieved simply by folding the optical axis with a folding mirror. It is therefore difficult to prevent a drop in image quality and reduce the size of the apparatus at the same time.
Although FIG. 9 of Patent Document 3, for example, shows a folding mirror folding the optical axis, where the spread of the light flux is prevented by a lens, the aforementioned problem of the quality of the projected image being subject to even a slight error in the shape of the folding mirror and thus tending to degrade still remains.